Chromium is an unavoidable trace element of the raw material used in the manufacture of cement, which is interground to produce hydratable cement. In the oxidizing and alkaline burning conditions of the cement kiln, hexavalent chromium (VI) may form. Hexavalent chromium is a powerful dermal irritant that is considered extremely toxic due to hits high oxidation potential and ability to penetrate human tissue. It can cause skin sensitization, allergic reactions, and eczema.
Hexavalent chromium has high solubility in water, and is released when cement is mixed with water. Thus, wet cement poses a health issue for workers who come into contact with wet cement or concrete.
A typical approach is to employ ferrous (iron II) sulfate to reduce hexavalent chromium (VI) to trivalent chromium (III), which tends to precipitate from solution, thereby reducing its risk as a dermal irritant.
However, ferrous sulfate has poor dosage efficiency in reducing chromium VI to chromium III. It has perhaps ten times the stoichiometric requirement when compared to stannous sulfate, another chromium reducer used during cement manufacturing. The poor dosage efficiency of ferrous sulfate is explained partly by the fact that it readily oxidizes from Iron (II) to Iron (III) during prolonged contact with air and water. The ferrous ion is known to react rapidly with oxygen to form ferric ion under alkaline conditions. (See e.g., Cotton, F. A.; Wilkinson, G., Comprehensive Inorganic Chemistry, 4th Ed., John Wiley and Sons, 1980, p 490.)
The high temperature and humidity levels of cement manufacturing plants especially render the ferrous sulfate highly susceptible to oxidative attack by atmospheric oxygen. Hence, plant operators sometimes use up to three times the amount of ferrous sulfate otherwise needed for reducing levels of chromium VI. They must also use their ferrous sulfate inventory quickly, due to its lack of storage stability, or it may become useless in short time.
U.S. Pat. No. 4,784,691 taught that one can protect the chromium-reducing ability of ferrous sulfate by coating particles with an oxidation-preventing material. However, this step introduces additional costs. It also does not decrease the dosage requirement to reasonable levels. This patent suggested, in Example 1, that up to thirty times the stoichiometric amount of ferrous sulfate would be required for dosage into cement.
Using large amounts of ferrous sulfate in cement to reduce hexavalent chromium levels creates a number of problems. When used in excess of 0.5 percent based on weight of the cement, ferrous sulfate tends to increase water demand and cement setting time. When used in dry powder form and in large amounts, ferrous sulfate is difficult to interblend uniformly within the cement, thereby exacerbating dosage inefficiency problems.
In view of these concerns, novel methods and compositions are needed for reducing hexavalent chromium levels during the manufacturing of cement and for use generally in hydratable cementitious compositions.